Twin-sheet thermoforming of plastic fuel tanks

ABSTRACT

A method and a plant for twin-sheet thermoforming of fuel tanks; first and second sheets (SA, SB) of thermoformable plastic material, are separately fed along respective processing lines ( 10 A,  10 B). The sheets (SA, SB) are heated and gripped along their peripheral edges by a pneumatically actuate suction and vacuum holding device ( 15 A,  15 B), for supporting the heated sheets (SA, SB) in a substantially flat condition while they are moved towards a respective thermoforming station ( 16 A,  16 B). Both the molds ( 17 A,  17 B) are disposed side by side with their open cavity facing upwards. After thermoforming of the plastic sheets (SA, SB), one of the molds ( 17 B) is turned upside down onto the other mold ( 17 A), to overlap and weld superimposed sealing areas of the two thermoformed shells (GA, GB). Cooling of the molds and thermoformed tank may be performed on a side of the processing lines ( 10 A,  10 B).

FIELD OF THE INVENTION

This invention refers to the thermoforming of plastic fuel tanks havinghigh structural features and gas barrier, starting from layered sheetsof plastic material, such as high-density polyolefin with EVOH barrierfor containment of fuel vapors; in particular the invention relates to amethod for manufacturing fuel tanks by “twin-sheet” thermoformingtechnology, and relative plant.

PRIOR ART

Metal fuel tanks are widely used in several fields, for example forsupply fuel to internal combustion engines, both in terrestrial andnautical vehicles and in aircrafts, as well as for other uses; however,the metal fuel tanks currently in use are heavy, difficult to shape andsubjected to corrosion.

The current tendency is to replace the metal fuel tanks with fuel tanksin layered plastic material, due to their greater lightness and abilityto withstand to corrosion, and the possibility to be manufactured bydeep drawing in complex shapes.

The rules presently in force, however, tend more and more to reduce thequantity of gaseous emissions and fuel vapors from the tanks, in orderto reduce environmental pollution. For such purpose, the possible gasand vapor emission paths through apertures in the walls of the fueltanks, necessary for connection to fittings and component parts, must bereduced to a minimum. In addition, the current approach is to install alarge part of the components and accessories necessary for feeding thefuel to an engine or user, inside the same fuel tank.

In the manufacture of fuel tanks in plastic material, use generally ismade of blow molding technology, according to which a parison or atubular element of plastic material is extruded between two half molds,which are subsequently clamped for peripherally pinching the parisondisposed between them; a pressurised fluid is then injected into theparison to cause the expansion and adhering the internal surfaces of themold.

In fuel tanks manufacturing by blow molding technology, after theshaping step it is necessary to make some apertures in the walls of thetank to install the various components, both inside and outside thetank. All this involves extremely complex solutions, long and somewhatexpensive manufacturing procedures, as well as a high risk of emissionof hydrocarbons through apertures in the fuel tanks, in the event notbeing accurately sealed. Moreover, whenever it is necessary tomanufacture fuel tanks by multi-layered material, any control of thewall thickness, proves to be extremely difficult to achieve.

In order to partially obviate these drawbacks, and to achieve aquantitatively high output, U.S. Pat. No. 6,372,176 and WO 02/14050propose the use of the known twin-sheet thermoforming technology.

According to this technology, sheets of thermoplastic material areheated and processed in respective molding station in which eachindividual sheet of material is thermoformed in a shell, inside acorresponding mold; two shells are subsequently joined and sealed aroundtheir peripheral edges to form a fuel tank.

According to said documents, first and second sheets of plastic materialare heated and processed along independent processing lines where eachplastic sheet is moved from a loading station, to a thermoformingstation in which each preheated sheet is thermoformed in a respectivefemale mold; a first one of the molds is mounted in a facing-updisposition on a lower platen, while the second mold is mounted in afacing down disposition on an upper platen of a press.

After the two sheets has been thermoformed, the upper mold must befirstly aligned to the lower mold and then lowered to apply a force tofuse and sealing the thermoformed shells along their peripheral edges.

WO 03/097330 also relates to an apparatus for thermoforming twin-sheethollow plastic articles in which again use is made of upwardly anddownwardly facing thermoforming molds.

Operators can introduce various inserts and/or components inpre-established positions, before the two shells are joined and sealedto form a tank.

Although the solutions proposed in the prior art documents enable thetwin-sheet thermoforming technology to be used for obtaining highproduction volumes, it is susceptible to further improvements tending toimprove both the manufacturing process, and the thermoformed fuel tanks.

In fact, according to prior-art documents, the thermoforming of thelower shell takes place by maintaining a facing-up disposition of thecavity of the lower mold, while the thermoforming of the upper shell iscarried out by maintaining a facing down disposition of the cavity ofthe upper mold.

All this entails considerable difficulties in thermoforming the uppershell, as well as structural unevenness in the fuel tank, due to adifferent sagging and stretching of the plastic sheets, in particular ofthe upper sheet in that the facing down cavity of the upper mold isopposite to the sag caused during the heating of the plastic sheet.

The different disposition and the different orientation of the twomolds, as well as the different stretching conditions of the two plasticsheets caused by the sag, consequently entail structural unevenness andthickness differences in the two shells, which are difficult toeliminate.

Moreover, the different disposition of the molds in the two processinglines complicates the insertion of components into the fuel tank, makingthe installation extremely cumbersome and difficult for an operator togain access in order to carry out the necessary checks and inspections.Replacement of the molds and the fixtures or jigs is also made difficultto carry out.

In the manufacture of storage tanks in plastic material, in order toprovide the necessary structural resistance and the required hydrocarbongas-tightness or barrier, in general use is made of layered plasticmaterial by overlapping a number of plastic sheets having differentchemical and/or physical properties.

Since, in the manufacture of plastic fuel tanks, according to thecurrent technologies, large quantities of scraps are produced, and sincecertain costly plastic materials are difficult to recovery, there is thebig problem of finding new molding systems whereby, in addition toimproving the output, the scraps and, consequently, the molding costs ofthe tanks can be reduced to a minimum.

OBJECTS OF THE INVENTION

Therefore, the main object of this invention is to provide a method anda plant for thermoforming hollow bodies, in particular fuel tanks bymeans of the twin-sheet thermoforming technology, capable of providinggreater simplification of the manufacturing cycle, and of processing ina substantially identical way the plastic sheets designed to form thetwo shells of a fuel tank.

Another object of the invention is to provide a method and a plant asmentioned above, whereby it is possible to manufacture fuel tankscharacterised by a high degree of structural homogeneity.

A still further object of this invention is to provide a method and aplant for manufacturing fuel tanks by means of the twin-sheetthermoforming technology which, in addition to operating simultaneouslyon both processing lines, also make it possible to operate withextremely short production cycles, in a substantially continuous way,without downtimes, considerably simplifying the insertion of thecomponents into the molds.

Another object of this invention is to provide a method forthermoforming fuel tanks in plastic material, whereby it is possible tosubstantially reduce the problems related to recovery of the scraps, inthat it allows a smaller loss of valuable material.

A further object of the invention is to provide easy accessibility tothe forming area, as well as facilitating the maintenance operations forthe entire plant.

BRIEF DESCRIPTION OF THE INVENTION

According to the invention, all the above is achievable by means of amethod for twin-sheet thermoforming of fuel tanks according to claim 1,and with a plant according to claim 7.

More precisely according to the invention a method for thermoformingplastic fuel tanks has been provided, according to which first andsecond sheets of thermoformable plastic material are heated andindependently moved and processed, along a first, and respectively alonga second processing line, from a loading station to a thermoformingstation, the method comprising the steps of:

-   -   subjecting each plastic sheet to a heating;    -   pneumatically clamping the heated plastic sheet along its        peripheral edge, and vacuum supporting the same sheet in a        substantially flat condition while it is moved along the        respective processing line;    -   positioning each heated plastic sheet above a respective shaping        mold having a facing-up shaping cavity, while continuing to        pneumatically hold the sheet in the aforesaid substantially flat        condition;    -   lowering the heated plastic sheet into a respective mold; and    -   thermoforming each heated plastic sheet into a respective shell,        making the same sheet to adhere to the upwardly open cavity of        the shaping mold;

the method also comprising the supplementary steps of:

-   -   up-side down turning one of the shaping molds and the        thermoformed shell;    -   superimposing said up-side down turned mold to the other one        facing up mold, to overlap peripheral sealing areas of the two        superimposed thermoformed shells; and    -   fusing and hermetically welding the overlapped sealing areas of        the shells by pressing said overlapped sealing areas between        clamping surfaces of the shaping molds.

According to another feature, the invention relates to a twin-sheetthermoforming plant for the manufacture of plastic fuel tanks comprisingfirst and second twin-sheet thermoformed shells, according to whichfirst and second thermoformable plastic sheets are independently heatedand moved along respective first and second processing lines, from aloading station through at least one heating station, towards arespective thermoforming station where the individual plastic sheets arethermoformed in a first and a second shell into a respective first andsecond shaping mold, wherein:

-   -   said first and second shaping molds are side by side arranged        with the open cavities of both molds facing upwards;    -   wherein each processing line comprises pneumatically actuable        gripping means for gripping the plastic sheets around their        peripheral edges, and a vacuum sheet holding device for the        heated plastic sheets, said pneumatic gripping means and said        vacuum holding device being movable along the processing lines;        and vacuum control means for the vacuum holding device for        supporting the heated plastic sheets in a substantially flat        condition; and    -   drive means conformed and arranges to turn one mold upside down        to superimpose to the other one and to cause welding of        overlapped sealing areas of the thermoformed shells, by        compression of the same molds.

Before heating at the thermoforming temperature, each plastic sheet canbe subjected to a preheating, followed by a centering in a successivecentering station, to allow a correct pneumatic gripping of theperipheral edges of the sheet; if required, the centering may precedethe preheating step. In addition, by using special pneumatic frames, itis possible to substantially reduce the gripping surfaces of the sheetsand consequently the scraps of material.

According to a further feature of the invention, the substantialflatness of the plastic sheets, both during the heating and the movementof the heated sheet towards the thermoforming station, can be suitablycontrolled by changing the softening temperature of the plasticmaterial, and/or adjusting the vacuum inside the supporting device.

For the purposes of this specification, the term “substantially flat” isunderstood to mean a condition in which the heated sheet ispneumatically held on the peripheral edges, and is vacuum supported onthe upper side, without sagging downwards to any great degree, due tothe gravity.

According to another feature of the invention, the heating, centeringand thermoforming steps of the plastic sheets along the two processinglines, can be carried out cyclically, by feeding the individual sheetsalong the two processing lines while several operations are carried outsimultaneously, or in succession.

According to the invention, after the thermoforming and sealing of thetwo shells into a tank, it is possible to carry out a cooling step ofthe fuel tank in a separate cooling station, disposed at a side of theprocessing lines. Since the cooling of the molds and the thermoformedtank takes a considerable length of time, in this way it is possible tocool the tanks after the thermoforming, without stopping or affectingthe manufacturing process. This can be achieved by disposing, on oneside of one of the two processing lines, in correspondence with thethermoforming station, a cooling station comprising a rotary table or areciprocable shuttle having two or more mold supporting zones, which canfrom time to time be aligned with the thermoforming stations of theprocessing lines, to support the closed molds and the thermoformed tank.

The two closed molds with the tank can be simply transferred onto therotary table or onto the supporting shuttle. Consequently, whenever thesealing of the two shells and the cooling of the storage tank take placeby feeding a pressurised fluid into the same tank, in correspondencewith the cooling station it is necessary to make use of a suitableclamping press to maintain the tank closed into the two molds.Optionally, it is possible to make use of a special cage for containingthe closed molds, as an alternative to the press in the cooling station.

According to a further feature of the invention, a method and a plantfor twin-sheet thermoforming of storage tanks have been provided,whereby the two processing lines can be disposed and parallely arrangedwith both facing up molds at a same level; this greatly facilitates theaccess of one or more operators to the entire plant, for the necessarycontrols, such as for introduction of components into the two shells, aswell as for all the necessary maintenance operations. Replacement of themolds is also greatly facilitated and can be carried out outside of thetwo processing lines, in correspondence with the cooling station.

BRIEF DESCRIPTION OF THE FIGURES

These and further advantages and features of the method and plantaccording to this invention, will be more clearly evident from thefollowing description, with reference to the accompanying drawings, inwhich:

FIG. 1 shows a block diagram of the various steps and working stations,according to a preferential embodiment of the invention;

FIGS. 2A-2H show flow charts illustrating the various operations carriedout along each processing line;

FIGS. 3 and 4 show a top view and respectively a side view of a bookpress of a thermoforming station, in an open condition;

FIG. 5 shows a view of a vacuum bell for pneumatically gripping andvacuum supporting the plastic sheets along each processing line;

FIG. 6 shows an enlarged detail of FIG. 5;

FIG. 7 shows a top view of the plant, in correspondence withthermoforming station, according to a first embodiment;

FIG. 8 shows a view similar to that of the previous figure, according toa further embodiment;

FIGS. 9 and 10 show a front view and a side view of a cage in a closedand an open conditions, for containing the closed molds during cooling.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the twin-sheet thermoforming of fuel tanks takesplace by processing individual sheets of thermoformable plastic materialalong two separate processing lines 10A and 10B, which parallely extendfrom a loading station for the plastic sheets, to a thermoformingstation through intermediate work stations; along the two lines 10A,10B, the individual plastic sheets are heated and shaped in respectivethermoformed shells, by subjecting the plastic sheets to a sameprocessing steps, while they are moving in pairs through the variouswork stations of a plant.

For the purposes of this description, “thermoformable plastic sheet” isunderstood to mean any plastic material in sheet form, suitable forbeing shaped by a thermoforming process; plug assist may also beprovided. The plastic sheets can either be of a single layer of suitablethickness, or layered, that is to say composed of several layers ofplastic material of identical and/or of different thickness, havingchemical and/or physical properties differing from one another.

As shown in FIG. 1, each processing line 10A, 10B comprises a number ofwork stations in which the various processing steps are carried out; inparticular, in a first station 11A, 11B, a first loading step takesplace, by picking up the individual sheets SA, SB from a pallet.

The individual plastic sheets SA, SB are picked up automatically and fedinto the loading stations 11A, 11B by means of mechanical and/orpneumatic gripping devices; then are transferred to a preheating station12A, 12B where the plastic sheets remain in a heater for the timenecessary to bring them at a first preheating temperature lower than thethermoforming temperature of the same sheets. Even though it ispreferable to preheat the plastic sheets SA, SB in order to reduce theworking cycle times, the preheating phase may also be omitted.

The preheating temperature for the plastic sheets SA, SB is suitablycontrolled, for example, by making the sheets move along a heating pathinside an air convection heater, in which the sheets remain for aheating period equivalent to several processing cycles, during which theindividual sheets are heated gradually to a desired temperature.

Upon completion of the preheating step, each individual sheet SA, SB istransferred to a subsequent centering station 13A, 13B, where the sheetis positioned and oriented to be pneumatically picked up, as explainedfurther on.

In the case of FIG. 1, the centering step 13A, 13B is carried outdownstream, immediately after the preheating; in certain cases, thiswould imply having to maintain a comparatively low preheatingtemperature with respect to the thermoforming temperature, therebyincreasing the final heating time. In order to obtain better centeringconditions for the sheets, the latter may be preliminarily centred,prior to their introduction into the preheating oven; this embodimentoffers the possibility of increasing the preheating temperature andconsequently reducing the final heating time.

According to the exemplificative diagram of FIG. 1, after the pre-heatedsheets have been centred in the stations 13A, 13B, the individual sheetsare transferred to a subsequent final heating station 14A, 14B wherethey are heated is to a temperature close or equivalent to athermoforming temperature, depending upon the properties of the plasticmaterial or materials of which each individual sheet SA, SB is composed.

In this connection, as schematically indicated in FIG. 1, the individualsheets SA, SB in the centering stations 13A, 13B are picked up by meansof a suitable pneumatic gripping device 15A and 15B, movable above eachprocessing line, for example of the type shown in FIGS. 5 and 6.

FIG. 5 shows, by way of example, a longitudinal cross-sectional view ofthe pneumatically actuate gripping and vacuum holding device 15A; thepneumatic gripping device 15B is wholly similar to the device 15A.

As shown, the pneumatic gripping and vacuum holding device 15A isprovided by of a bell-shaped device having peripheral walls 21 forming afacing down vacuum chamber 22; the vacuum chamber 22 is provided with acone-shaped upper portion ending with a fitting 23 for connection to anair suction source 23′, necessary for creating a sufficient vacuumdegree in the vacuum chamber 22 for supporting or holding the plasticsheet material SA in a substantially flat condition; by controlling thesuction source 23′, it is possible to control the vacuum degree in thevacuum chamber 22 of the bell-shaped device 15A and consequently theflatness of the plastic sheet SA.

The shape and size of the device 15A substantially correspond to thoseof the plastic sheets SA, SB to be thermoformed. Consequently, thebell-shaped device 15A in FIGS. 5 and 6, is provided at the lower edge,with pneumatically actuable gripping means for gripping the sheet SAaround its peripheral edge, forming an adequate airtight seal.

As shown in FIG. 5 and in the enlarged detail of FIG. 6, the pneumaticgripping means for gripping the sheet SA may comprise a flat peripheralframe 24 secured inside and spaced apart from the lateral walls 21 so asto form a slot 25 connectable in a controlled way to an air suctionsource 25′.

The pneumatic gripping means for gripping the plastic sheets canobviously be differently shaped or conformed, compared to those shown.Lastly, also in FIG. 5, it can be seen that the bell-shaped device 15Afor gripping and vacuum supporting the sheets SA, SB, is provided with abank 28 of heating elements for heating the sheets SA, SB, within thesame vacuum chamber 22.

Picking up and moving each individual preheated sheet SA, SB,constitutes one of the most delicate steps of the entire process, inthat, if the heated sheets were not properly gripped and supported, dueto the gravity they would tend to sag downwards, and undergouncontrolled deformation which would have a negative effect on thesubsequent thermoforming of the shells in the stations 16A, 16B.

Therefore, according to a preferential embodiment of the invention, theindividual sheets SA, SB in the respective centering stations 13A, 13Bare picked up pneumatically in a controlled way at their peripheraledges, and vacuum supported by means of the bell-shaped devices 15A, 15Bof FIG. 5, maintaining them in a substantially flat condition while theyare being transferred to the heating stations 14A, 14B. In thesestations, the individual sheets SA, SB are further heated to bring themup to a temperature equivalent or close to the thermoformingtemperature. The heating of the sheets SA and SB in the two heatingstations 14A and 14B may takes place on both sides from below by meansof a bank of heating elements 47 which each station 14A, 14B is providedwith, and from above by means of the bank of heating elements 28 insidethe bell-shaped device 15A and 15B.

After the heating step at the thermoforming temperature has beenperformed in the stations 14A, 14B, the heated plastic sheets SA, SB aretransferred by means of the devices 15A, 15B to the subsequent stations16A, 16B to be subjected to the thermoforming into the molds 17A, 17Bfor shaping them into corresponding thermoformed shells GA, GB accordingto the procedure illustrated further on in FIGS. 2A-2H of theaccompanying drawings.

As mentioned previously, the transfer of the heated sheets SA, SB takesplace while supporting them by vacuum in a flat or substantially flatcondition, that is to say, devoid of deep sagging, by means of thebell-shaped devices 15A, 15B which can therefore be moved rapidlybetween the various in-line work stations.

In order to maintain a substantial flattened condition of the heatedsheets, preventing the latter from sagging excessively as the heatingtemperature gradually increases, the sagging of the sheets is detectedand the vacuum degree created above the heated sheets by the devices15A, 15B, is constantly controlled and adjusted so as to prevent anexcessive sagging, with the risk of the plastic sheets striking againstfixed parts of the plant, preventing or hindering their correctpositioning above the molds 17A, 17B.

As shown in greater detail further on, the essential feature of thisinvention resides in thermoforming the plastic sheets SA, SB whilemaintaining substantially identical thermoforming conditions for both ofthem, so as to obtain two structurally homogeneous thermoformed shellsGA, GB.

This can be obtained, according to the invention, by disposing both themolds 17A, 17B side by side at a same level, with the respective shapingcavities facing upward.

The side-by-side and upwardly oriented disposition of the twothermoforming molds, enables the sheets to be also pre-stretched anddeposited on the molds, simply by gravity. This not only simplifies thethermoforming enormously, making it possible to obtain the desiredstructural homogeneity of the two shells GA, GB, but also facilitatesthe operations of introducing inserts and components into one or boththe shaped shells before sealing the tank. In fact, thanks to the upwardoriented disposition of the cavities for both the molds 17A, 17B, it ispossible to carry out the introduction of the inserts and/or the variouscomponents, both prior and subsequent to the thermoforming, directly ineach mold or into the thermoformed shells, while the latter still remainin the respective mold, picking up by an operator the various componentsfrom a side station 18A and 18B, as shown in greater detail in FIGS. 7and 8.

Upon completion of the thermoforming of the two shells and theintroduction of the various components, as mentioned previously, stillin one of the thermoforming stations a subsequent step may be carriedout consisting in superimposing the two molds 17A, 17B, fusing andsealing the two thermoformed shells at overlapped sealing areas, asschematically shown by the block diagram 19 in FIG. 1.

This can be achieved in any suitably way; for example, by upside downturning one of the molds onto the other one, by simple rotation aroundan horizontal axis, or in any other way capable of allowing thesuperimposition of the two molds with the respective thermoformingshells, one turned upside down on top of the other and with theirperipheral sealing area matching.

In this connection, as schematically shown in FIGS. 3 and 4, use can bemade of a book-like press.

The thermoforming press comprises a stationary frame 30 and a movableframe 31 which can be overturned by 180° around a rotational axis 32connected to a drive motor 33, or in any other suitable way.

The stationary frame 30 of the press is in turn provided with a platen34 for supporting the mold 17B; the platen 34 is vertically movable bymeans of hydraulic cylinders 35 for providing the necessary clampingforce for closing the molds 17A, 17B and sealing the overlapped areas ofthe two shells, in the closed condition of the molds shown by the blockdiagram 19 in FIG. 1.

From FIGS. 3 and 4 it can also be seen that, according to anotherfeature of the invention, pneumatically actuable sheet gripping meansare provided for each mold 17A, 17B; the gripping means comprises apneumatic frame 36A, 36B for gripping the sheets SA, SB from below,around their peripheral edges, on the side oppsite to that of thegripping device 24 of the bell-shaped device 15A, 15B as explainedfurther on. In this connection the pneumatic frames 36A, 36B are shapedand sized identically to the pneumatic frame 24 of two bell shapeddevices 15A and 15B. This proves to be extremely advantageous in that itenables the individual sheets SA, SB to be supported along the twoprocessing line 10A, 10B and transferred by the bell shaped devices 15A,15B above the shaping molds 17A, 17B, by gripping the sheets themselvesalong a narrow peripheral strip. This makes it possible to considerablyreduce scraps, and losses of valuable material, compared to theconveying systems previously used in the usual thermoforming plantswhich make use of belts or other similar conveying systems for theplastic sheets.

Each pneumatic frame 36A, 36B, can be vertically moved upwards, withrespect to the mold 17A, 17B, by means of suitable control cylinders37A, 37B. The pneumatic gripping frames 36A and 36B can be shaped with aflat or variable profile, by providing articulated frame sections, toadapt to molds with flat or three-dimensional shaped edges; moreover,each frame can be provided also with mechanical gripping means forsecuring the edges of the individual sheets.

Once the two molds 17A, 17B with the corresponding thermoformed shellshave been placed one on top of the other, the peripheral sealing areasof the two shells are pressed and fused one against the another bysealing edge zones of the thermoforming molds themselves, thereby fusingand hermetically sealing a fuel tank.

Upon completion of the thermoforming and sealing of the two shells, itis necessary to perform a cooling step for cooling the tank, beforeremoving the same from the molds.

The cooling can be carried out in any suitable way, for example bycirculation of water or a cooling fluid within the mold walls, or bycirculation of air while maintaining the tank under pressurisedcondition to urge against the thermoforming molds.

Although the cooling can be carried out directly along the processingline, in one of the two thermoforming stations 16A, 16B, since thecooling of the molds and the fuel tanks takes a considerable length oftime, according to another feature of this invention it is preferable tocarry out the cooling separately from the processing line; in thisconnection, the closed molds with the thermoformed tank are transferredto a cooling station 20, on a side of the processing lines 10A, 10B. Byremoving the closed molds and carrying out the cooling in a sidestation, separate from the processing lines, it is possible to operatein a continuous cycle, without causing delays or dead times in themanufacturing process, which can continue as explained further on. Inaddition, by removing the molds from the processing lines andtransferring them to a separate cooling station, it is possible toeasily work on the same molds for maintenance operations and/or fortheir replacement.

After the fusing and sealing of the two shells, depending on whether thecooling is to be carried out with the fuel tank under pressurisedconditions or not, the closed molds must be transferred from one of thethermoforming stations to the cooling station.

In the event the cooling being carried out with the tank underpressurised conditions, in order to counteract the pressure inside themolds, it is advisable to close the latter in a special retaining cage,shown in FIGS. 9 and 10 of the accompanying drawings.

For example, as shown in FIGS. 9 and 10, the cage may comprises a lowerplaten 40 and an upper platen 41 at which are secured the two molds 17A,17B. The upper platen 41 is articulated to two lateral posts 42 torotate around an articulation axes 43 capable to vertically slide alongan elongated hole 42′. The rotational movement of the upper platen 41 iscontrolled by two hydraulic cylinders 44, while wedges 45 actuated byhydraulic cylinders 46 enable the upper platen 41 to be locked to thetwo lateral posts 42, in the closed condition of FIG. 9.

With reference to the diagram of FIGS. 2A-2H, a more detaileddescription is given hereunder of the operating method of the twin-sheetthermoforming plant according to this invention.

The FIGS. from 2A to 2H schematically show the individual operativesteps which are carried out in parallel and in succession in the variouswork stations along the two processing lines, for example between thecentering station 13A of the line 10A, and the thermoforming station16A, it being clearly understood that the same processing steps will besimultaneously carried out in parallel on the other processing line 10B.

As mentioned initially, the individual plastic sheets SA in the loadingstation 11A, are picked up and introduced into the oven 12A where theyare held for a pre-established period of time, in the region of a fewdozen minutes, depending upon the nature and the properties of theplastic material, making them undergo a gradual preheating to a firsttemperature lower than the thermoforming one.

After completion of the preheating phase, at the outlet of the oven 12Athe preheated sheet SA is transferred to the centering station 13A, forexample by means of a roller table 45 or other conveying system, wherean appropriate centering device 46 (FIG. 2A) is raised for accuratelypositioning and orientate each sheet SA1, to be perfectly aligned withthe pneumatic gripping device 15A which in the meantime has been movedabove the centering station 13A.

Upon completion of the centering of the sheet SA1, the latter ispneumatically gripped around its peripheral edge and raised by means ofthe pneumatic gripping device 15A which is supported to move verticallyand/or horizontally in the directions of the double arrows F2 and F3,above the processing line 10A.

The device 15A for gripping the sheets is conformed and operates forpneumatically picking up the sheet SA1 and for forming a seal around theperipheral edge so that above the sheet itself inside the bell 15A it ispossible to create, in a controlled way, a certain vacuum degree capableof holding the sheet SA1 in a substantially flat condition, as shown.

The pneumatic gripping device 15A with the sheet SA1 can consequentlymove from the centering station 13A, to the heating station 14A and thethermoforming station 16A, as schematically shown in FIGS. 2A, 2B, 2Cand 2D of the accompanying drawing.

In correspondence with the centering station 13A, the pneumatic grippingdevice 15A picks up the already centred sheet SA1, holding itpneumatically around the peripheral edge.

Immediately after, the heater 28 is switched on, the bell 15A isconnected to a vacuum source and then, from the centering station 13Athe pneumatic gripping device 15A with the preheated sheet SA1 is movedtowards the final heating station 14A, as shown in FIG. 2C.

During this step, the plastic sheet material SA1, likewise to the sheetSA2, is further heated to the required thermoforming temperature, bothby the upper heater 28 of the gripping device 15A, and by a lower heat47 positioned below in the same heating station 14A. Since during thisheating step the plastic material of the sheet is brought up to atemperature close to melting point, and would consequently tend to sagdownwards by gravity, the vacuum in the bell of gripping device 15A ismaintained and suitably controlled so as to keep the sheet SA1 in asubstantially flat condition, throughout the entire heating period untilit has been positioned above the thermoforming mold 17A.

In the meantime, several operators have taken care to introduce insertsinto the mold 17A, as schematically shown in FIG. 2C.

The plastic sheet SA1 is then heated on both sides in a whollycontrolled way. Upon completion of the heating, the gripping device 15Ais quickly moved above the thermoforming mold 17A, as shown in FIG. 2D.

The gripping and vacuum holding device 15A with the sheet SA1 heated tothe thermoforming temperature, is transferred while maintaining theheating by means of the heater 28, and the vacuum, for the reasonsexplained previously.

At this point, by reducing the vacuum, or controlling the pressurewithin the gripping and holding device 15A, it is possible to form a sagby gravity, by making the sheet sag downwards, as shown in FIG. 2E;simultaneously, the second gripping device 36A associated with the mold17A, is raised to grip the sheet SA1 from below, around the peripheraledge, on the side opposite to that of the upper gripping frame 24 of thegripping and holding device 15A, as again shown in FIG. 2E.

In the meantime, a new preheated sheet SA2 can be transferred to thecentering station 13A.

At this point, the device 15A releases the sheet SA1 which isimmediately gripped from below by the second gripping device 36A of thethermoforming mold 17A; the first gripping and sheet holding device 15Anow can be made to return towards the centering station 13A where it canpick up a second sheet SA2, as shown in FIG. 2F.

The same FIG. 2F shows that the second gripping device 36A which holdsthe first sheet SA1 in a sagging condition, is lowered, and stopped at adistance from the mold 17A, while a shaping plug 49 is made to adherefrom the above to the sag of the sheet SA1.

Both the shaping plug 49 and the gripping device 36A are now loweredsimultaneously bringing the sheet SA1 to rest against the internalsurface of the mold 17A. By activating vacuum in the mold 17A, in a perse known way, the sheet SA1 is shaped into a corresponding thermoformedshell by the combined action of the vacuum inside the mold 17A and thepressure of the shaping plug 49.

During the thermoforming of the shell, the peripheral edge of theplastic sheet SA1 is retained against the peripheral edge of the mold17B, for example by means of a presser 48, or in any other suitable way,as shown in FIG. 2G.

Upon completion of the thermoforming of the shell, when the plasticmaterial is still at a high temperature, the plug 49 is raised to allowthe introduction of any further inserts or components into thethermoformed shell. All this can be carried out quickly and easilythrough the upward facing of the molds, thereby greatly simplifying allthe operations on both processing lines.

As mentioned previously, all the operations described with reference tothe FIGS. from 2A to 2H, are carried out simultaneously and cyclicallyon two plastic sheets SA1, SB1 along the two processing lines 10A and10B. Consequently, in both cases, the thermoforming of the two shellstakes place in a substantially identical way, with both the molds facingupwards; moreover, in both cases, vacuum and gravity are appropriatelyused for controlling the formation of the sag in each of the two sheetsSA1, SB1.

It is now necessary to seal the two shells to form a fuel tank; in thisconnection, the hermetical sealing of the two thermoformed sheets SA1and SB1 is carried out by tightly pressing and fusing their sealingareas one against the other around the peripheral edges of the twoshells.

This can be achieved, for example, by overturning one of the two moldsby 180°, for example by making the mold 17B rotate in the direction ofthe arrow F1 in FIG. 4, around the rotational axis 32, upside down theother mold 16A, as shown by the block diagram 19 in FIG. 1.

At this point, the two molds are tightly pressed one against the other,exerting an adequate clamping force, and pinching the two sealing areasof the two shells thereby fusing them hermetically together.

After having completed a fuel tank, the latter can be cooled whilemaintaining it closed in the two molds. Upon completion of the cooling,the two molds, still closed, can be moved back to one of the twothermoforming stations 17A, 17B and opened to carry out the demoldingand discharge of the finished fuel tank, directly in the samethermoforming station.

All this can be carried out within an extremely short period of time,during the execution of a subsequent operative cycle, so as to use thesame molds again for manufacturing a next fuel tank.

Therefore, by operating in a continuous cycle with the two processinglines 10A and 10B, according to the previously described procedures, itis possible to produce plastic fuel tanks, complete with the respectiveaccessories and components, in an extremely limited time, reducing thescraps of material due to the combined use of the two pneumatic grippingand conveying devices along the two processing lines, makes it possibleto form sealing having extremely limited surfaces; according to theinvention it is also possible to produce plastic fuel tanks completewith their respective accessories having improved structuralcharacteristics.

FIGS. 7 and 8 show, by way of example, two possible solutions withregard to the cooling station, in addition to other details of thethermoforming stations and the stations for introducing the insertsand/or components into the molds and/or into the two thermoformedshells; in said figures, the same reference numbers used as per theprevious figures, to indicate similar or equivalent parts.

As shown in FIG. 7, the cooling station 20 comprises a rotary table 50having two or more molds supporting surfaces 51, 52, onto which theclosed molds 17A, 17B are transferred for the cooling phase.

The rotary table 50 is made to rotate and indexed by a motor 53 in orderto align from time to time one of the mold supporting surfaces 51, 52,with the thermoforming station, to allow the transfer of the closedmolds, for example, between the thermoforming station 16B and thesurface 51, while another closed molds undergoing cooling is alreadydisposed on the surface 52 or on the other surfaces of the rotary table.FIG. 7 schematically indicates various robots or operators for themovement of the shaping plug 49, or for picking up and automaticallyintroduce the inserts into the molds, or for discharging the finishedfuel tanks.

The example of FIG. 8 differs from the previous figure in that thecooling station 20 now comprises a shuttle 54 reciprocable along a guide55, which extends on a side parallel to the processing line 10B. Theshuttle 54 is provided with two mold supporting surfaces 56, 57 whichcan be aligned with the thermoforming station 16B, by appropriatelymoving the shuttle along the guide 55.

In both cases, the mold supporting surfaces 51, 52 of the rotary table50, or the mold supporting surfaces 56, 57 of the shuttle 55, can beused for carrying out the replacement of the molds, at each change ofproduction, or for other requirements.

It is understood that what has been described with reference to theaccompanying drawings, has been given purely by way of example in orderto illustrate the general features of the method and the plant accordingto this invention; therefore, other modifications or variations may bemade, without thereby deviating from the scopes of the claims.

1. A plant for manufacturing plastic fuel tanks comprising first andsecond twin-sheet thermoformed shells (GA, GB), according to which firstand second thermoformable plastic sheets (SA, SB) are independentlyheated and moved along respective first and second parallely arrangedprocessing lines (A, B), from a loading station (11A, 11B) through atleast one heating station (12A, 14A; 12B, 14B), towards a respectivethermoforming station (16A, 16B) where the individual plastic sheets(SA, SB) are thermoformed in a first and a second shaping mold (17A,17B) into a first and second respective shell (GA, GB), wherein: saidfirst and second shaping molds (17A, 17B) are side by side arranged withthe open cavities of both molds (17A, 17B) facing upwards; wherein eachprocessing line (A, B) comprises a pneumatically actuable gripper formedfrom an air suction frame (24) configured for gripping on a side of theplastic sheets (SA, SB) around their peripheral edges, and a vacuumsheet holding device (15A, 15B) including a vacuum chamber (22) forholding the heated plastic sheets (SA, SB), said pneumatic gripper (24A)and said vacuum sheet holding device (15A, 15B) being movable along theprocessing lines (A, B); and a vacuum control device including anadjustable vacuum source to control a vacuum degree in the vacuumchamber (22) of the vacuum sheet holding device (15A, 15B) forsupporting the heated plastic sheets (SA, SB) in a substantially flatcondition; a pneumatic gripping device comprising said air suction frame(24), a corresponding pneumatic gripping frame (36A, 36B) movinglysupported by control cylinders (37A, 37B) on each mold, and a drive (33)conformed and arranged to turn one mold of molds (17A, 17B) upside downto superimpose to the other mold of molds (17A, 17B) and to causewelding of overlapped sealing areas of the thermoformed shells (GA, GB),by compression of the overlapped sealing areas by the molds (17A, 17B),wherein sheet transfer device (15 a) includes a vacuum box or bellelement defining a bottom open vacuum chamber, movable up and down inrespect to the processing line, and along the same processing line, thevacuum box or bell element is provided with a pneumatic clamping devicehaving at least one air suction channel peripherally arranged around theedges of the vacuum box or bell element, the air suction channel isconnectable to an air suction source to clamp the sheet SA at its upperside along its peripheral edge, and to close the vacuum chamber bytightly clamping the plastic sheet SA, and the vacuum box is connectableto the vacuum source by vacuum control means for vacuum holding theheated sheet SA in a suspended, horizontal and substantially flatcondition, during movement of the transfer device (15 a) along theprocessing line.
 2. The plant for manufacturing plastic fuel tanksaccording to claim 1, comprising a sheet preheating station (12A, 12B).3. The plant for manufacturing plastic fuel tanks according to claim 1,comprising a sheet centering station (13A, 13B).
 4. The plant formanufacturing plastic fuel tanks according to claim 3, wherein the sheetcentering station (13A, 13B) is provided upstream of the sheetpreheating station (12A, 12B).
 5. The plant for manufacturing plasticfuel tanks according to claim 3, wherein the centering station (13A,13B) is provided between the preheating station (12A, 12B) and a secondheating station (14A, 14B) for the plastic sheets (SA, SB).
 6. The plantfor manufacturing plastic fuel tanks according to claim 1, wherein thevacuum sheet holding device (15A, 15B) comprises heating elements (28)for the plastic sheets (SA, SB).
 7. The plant for manufacturing plasticfuel tanks according to claim 1, wherein said vacuum sheet holdingdevice (15A, 15B) is in the form of a pneumatically actuable suctionbell.
 8. The plant for manufacturing plastic fuel tanks according toclaim 1, comprising a mold cooling station (20) on one side of theprocessing lines (A, B), in a side aligned condition with athermoforming station (16A, 16B), the mold cooling station (20) beingconfigured for transferring the closed molds (17A, 17B) between thethermoforming station (16A, 16B) and the cooling station (20) of theplant.
 9. The plant for manufacturing plastic fuel tanks according toclaim 8, wherein the cooling station (20) comprises a rotary table (50)having a plurality of mold supporting surfaces (51, 52).
 10. The plantfor manufacturing plastic fuel tanks according to claim 8, wherein thecooling station (20) comprises a reciprocable mold supporting shuttle(54) parallely arranged to the processing lines (A, B), said shuttle(54) being provided with at least a first and a second mold supportingsurfaces (55, 56).
 11. The plant for manufacturing plastic fuel tanksaccording to claim 8, wherein a device for transferring the molds (17A,17B) comprise a mold clamping cage (40, 41) reciprocable between athermoforming station (16A, 16B) and the cooling station (20).
 12. Theplant for manufacturing plastic fuel tanks according to claim 1, whereinsaid drive means for upside down turning one mold (17B), comprises abook press.